Honeycomb filter and ceramic filter assembly

ABSTRACT

A ceramic filter assembly having improved exhaust gas processing efficiency. The ceramic filter assembly ( 9 ) is produced by adhering with a ceramic seal layer ( 15 ) outer surfaces of a plurality of filters (F 1 ), each of which is formed from a sintered porous ceramic body. The seal layer ( 15 ) has a thickness of 0.3 mm to 3 mm and a thermal conductance of 0.1 W/mK to 10 W/mk.

TECHNICAL FIELD

[0001] The present invention relates to a honeycomb filter and a ceramicfilter assembly, and more particularly, to a honeycomb filter-formedfrom a sintered ceramic body and an integral ceramic filter assemblyproduced by adhering a plurality of honeycomb filters to one another.

BACKGROUND ART

[0002] The number of automobiles has increased drastically this century.As a result, the amount of gas discharged from automobile engines hascontinued to increase proportionally. Various substances suspended inthe exhaust gas that is emitted, especially from diesel engines, causepollution and severely affect the environment. Further, recentlyreported research results have shown that the fine particles suspendedin gas emissions (diesel particulates) may cause allergies or decreasesperm counts. Thus, actions to eliminate the fine particles suspended ingas emissions must immediately be taken for the sake of mankind.

[0003] Due to this situation, many exhaust gas purification apparatuseshave been proposed in the prior art. A typical exhaust gas purificationapparatus includes a casing, which is located in an exhaust pipeconnected to an exhaust manifold of an engine, and a filter, which isarranged in the casing and has fine pores. In addition to a metal or analloy, the filter may be formed from ceramic. A cordierite honeycombfilter is a known example of a ceramic filter. Recent filters are oftenformed from sintered porous silicon carbide body that is advantageousfrom the viewpoints of heat resistance and mechanical strength, has ahigh accumulating efficiency, is chemically stable, and has a smallpressure loss.

[0004] The pressure loss refers to the difference between the pressurevalue taken upstream of the filter and the pressure value takendownstream of the filter. A main cause of power loss is the resistancethe exhaust gas encounters when passing through a filter.

[0005] The honeycomb filter includes a plurality of cells extendingalong the axial direction of the honeycomb filter. When the exhaust gaspasses through the filter, the walls of the cells trap fine particles.This removes fine particles from the exhaust gas.

[0006] However, the honeycomb filter, which is made of a sintered poroussilicon carbide body, is vulnerable to thermal impacts. Thus, largerfilters are liable to crack. Accordingly, a technique for manufacturinga large ceramic filter assembly by integrating a plurality of smallfilters has recently been proposed to prevent breakage resulting fromcracks.

[0007] A typical method for manufacturing a ceramic filter assembly willnow be discussed. First, ceramic raw material is continuously extrudedfrom a mold of an extruder to form an elongated square honeycomb moldedproduct. After the honeycomb filter is cut into pieces of equal length,the cut pieces are sintered to form a filter. Subsequent to thesintering process, a plurality of the filters are bundled and integratedby adhering the outer surfaces of the filters to each other with aceramic seal layer having a thickness of 4 to 5 mm. This completes thedesired ceramic filter assembly.

[0008] A mat-like thermal insulative material, made of ceramic fiber orthe like, is wrapped about the outer surface of the ceramic filterassembly. In this state, the assembly is arranged in a casing, which islocated in an exhaust pipe.

[0009] However, in the prior art, there is a shortcoming in that thefine particles trapped in the ceramic filter assembly do not burncompletely and some of the fine particles remain unburned. Accordingly,the efficiency for processing the exhaust gas is low.

[0010] Further, the honeycomb filter of the prior art has corners. Thus,there is a tendency of stress concentrating on the corners of the outersurface and chipping the corners. Further, the seal layer may crack andbreak the ceramic filter assembly from the corners thereby damaging theentire ceramic filter assembly. Even if the assembly does not break,there is a shortcoming in that leakage of the exhaust gas may decreasethe processing efficiency.

[0011] During usage of the filter assembly, a high temperaturedifference between the honeycomb filters may cause thermal stress tocrack the honeycomb filters and break the entire assembly. Thus, thestrength of each honeycomb filter must be increased to increase thestrength of the honeycomb filter assembly.

[0012] The prior art ceramic filter assembly as a whole has arectangular cross-section. Thus, the periphery of the assembly is cut sothat the assembly as a whole has a generally round or ovalcross-section.

[0013] However, the filter has a plurality of cells. Thus, if theperiphery of the assembly is cut, the cell walls are exposed from theperipheral surface subsequent to the cutting. This forms lands and pitson the peripheral surface. Thus, even if the assembly is accommodated inthe casing with the thermal insulative material attached to theperipheral surface of the assembly, gaps are formed in the longitudinaldirection of the filters. Thus, exhaust gas tends to leak through thegaps. This lowers the processing efficiency of the exhaust gas.

[0014] With regard to diesel particulates trapped in the honeycombfilter, it has been confirmed that particulates having a small diameterhave a high lung attaching rate and increase the risk to health. Thus,there is great need to trap small particulates.

[0015] However, when the pore diameter and the porosity of the honeycombfilter are small, the honeycomb filter becomes too dense and hinderssmooth passage of the exhaust gas, which, in turn, increases thepressure loss. This lowers the driving performance of the vehicle,lowers fuel efficiency, and deteriorates the driving performance.

[0016] On the other hand, if the pore diameter and porosity rate areincreased, the above problems are solved. However, the number ofopenings in the honeycomb filter becomes too large. Thus, fine particlescannot be trapped. This decreases the trapping efficiency. Further, themechanical strength of the honeycomb filter becomes low.

[0017] It is a first object to provide a ceramic filter assembly havingan improved exhaust gas processing efficiency.

[0018] It is a second object of the present invention to provide aceramic filter assembly having superior strength.

[0019] It is a third object of the present invention to provide aceramic filter assembly that prevents fluid leakage from the peripheralsurface.

[0020] It is a fourth object of the present invention to provide ahoneycomb filter having small pressure loss and superior mechanicalstrength.

SUMMARY OF THE INVENTION

[0021] A first perspective of the present invention is an integralceramic filter assembly produced by adhering with a ceramic seal layerouter surfaces of a plurality of filters, each of which is formed from asintered porous ceramic body. The seal layer has a thickness of 0.3 mmto 3 mm and a thermal conductance of 0.1 W/mK to 10 W/mk.

[0022] A second perspective of the present invention is an integralceramic filter assembly produced by adhering with a ceramic seal layerouter surfaces of a plurality of elongated polygonal honeycomb filters,each of which is formed from a sintered porous ceramic body. Roundsurfaces are defined on chamfered corners of the outer surface of eachhoneycomb filter, and the round surfaces have a curvature R of 0.3 to2.5.

[0023] A third perspective of the present invention is an integralceramic filter assembly produced by adhering with a ceramic-seal layerouter surfaces of a plurality of filters, each of which is formed from asintered porous ceramic body. The ceramic filter assembly includes aceramic smoothing layer applied to the outer surface of the assembly,which as a whole has a generally circular cross-section or generallyoval cross-section.

[0024] A fourth perspective of the present invention is an integralceramic filter assembly produced by adhering with a ceramic seal layerouter surfaces of a plurality of elongated honeycomb filters, each ofwhich is formed from a sintered porous ceramic body. A ratio L/S betweena filter length L in a flow direction of a processed fluid and a filtercross-section S in a direction perpendicular to-the flow direction is0.06 mm/mm² to 0.75 mm/mm².

[0025] A fifth perspective of the present invention is an integralhoneycomb filter assembly produced by adhering with a ceramic seal layerouter surfaces of a plurality of honeycomb filters, each of which has aplurality of cells defined by a cell wall and which purifies fluidincluding particulates with the cell wall. A specific surface area ofgrains forming the cell wall is 0.1 m²/g or more.

[0026] A sixth perspective of the present invention is an elongatedhoneycomb filter formed from a sintered porous ceramic body. A ratio L/Sbetween a filter length L in a flow direction of a processed fluid and afilter cross-section S in a direction perpendicular to the flowdirection is 0.06 mm/mm² to 0.75 mm/mm².

[0027] A seventh perspective of the present invention is a honeycombfilter formed from a sintered porous ceramic body. An average porediameter of the honeycomb filter is 5 to 15, m, an average porosity is30 to 50%, and the honeycomb filter has 20% or more of through pores.

[0028] An eighth perspective of the present invention is a honeycombfilter having a plurality of cells defined by a cell wall and purifyingfluid including particulates with the cell wall. A specific surface areaof grains forming the cell wall is 0.1 m²/g or more.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a schematic view showing an exhaust gas purificationapparatus according to a first embodiment of the present invention.

[0030]FIG. 2 is a perspective view showing a ceramic filter assembly ofthe exhaust gas purification apparatus of FIG. 1.

[0031]FIG. 3 is a perspective view showing a honeycomb filter of theceramic filter assembly of FIG. 2.

[0032]FIG. 4 is an enlarged cross-sectional view showing the mainportion of the exhaust gas purification apparatus of FIG. 1.

[0033]FIG. 5 is an enlarged cross-sectional view showing the mainportion of the ceramic filter assembly of FIG. 2.

[0034]FIG. 6 is an enlarged cross-sectional view showing the mainportion of a ceramic filter assembly of a first modified example.

[0035]FIG. 7 is a perspective view showing a honeycomb filter accordingto a second embodiment of the present invention.

[0036]FIG. 8 is an enlarged cross-sectional view showing the mainportion of a ceramic filter assembly.

[0037]FIG. 9 is an enlarged cross-sectional view showing the mainportion of a ceramic filter assembly according to a first modifiedexample.

[0038]FIG. 10 is a perspective view showing the honeycomb filteraccording to the first modified example.

[0039]FIG. 11 is a perspective view showing a honeycomb filter accordingto a second modified example.

[0040]FIG. 12 is a perspective view showing a honeycomb filter accordingto a third modified example.

[0041]FIG. 13 is a side view showing a ceramic filter assembly accordingto a third embodiment of the present invention.

[0042] FIGS. 14(a) to 14(c) are schematic perspective views illustratinga manufacturing process of the ceramic filter assembly of FIG. 13.

[0043]FIG. 15 is a side view showing a ceramic filter assembly accordingto a modified example.

[0044]FIG. 16 is a perspective view of a ceramic filter assemblyaccording to a fourth embodiment of the present invention.

[0045]FIG. 17 is a perspective view showing a filter of the ceramicfilter assembly 3 of FIG. 16.

[0046]FIG. 18(a) is a schematic cross-sectional view showing the filterof FIG. 17, and FIG. 18(b) is a schematic side view showing the filterof FIG. 17.

[0047]FIG. 19 is a perspective view showing a honeycomb filter providedwith a honeycomb structure according to fifth and sixth embodiments ofthe present invention.

[0048]FIG. 20 is a cross-sectional view showing the filter 59 of FIG. 19taken along line 20-20.

[0049]FIG. 21 is an enlarged cross-sectional view showing the mainportion of an exhaust gas purification apparatus.

[0050]FIG. 22 is a perspective view showing a ceramic filter assembly.

BEST MODE FOR CARRYING OUT THE INVENTION

[0051] A diesel engine exhaust gas purification apparatus 1 according toa first embodiment of the present invention will now be described withreference to FIGS. 1 to 5.

[0052] Referring to FIG. 1, the exhaust gas purification apparatus 1 isan apparatus for purifying the exhaust gas emitted from a diesel engine2, which serves an internal combustion engine. The diesel engine 2 has aplurality of cylinders (not shown). Each cylinder is connected to abranch 4 of an exhaust manifold 3, which is made of a metal material.Each branch 4 is connected to a single manifold body 5. Accordingly, theexhaust gas emitted from each cylinder is concentrated at one location.

[0053] A first exhaust pipe 6 and a second exhaust pipe 7, which aremade of a metal material, are arranged downstream to the exhaustmanifold 3. The upstream end of the first exhaust pipe 6 is connected tothe manifold body 5. A tubular casing 8 made of a metal material isarranged between the first exhaust pipe 6 and the second exhaust pipe 7.The upstream end of the casing 8 is connected to the downstream end ofthe first exhaust pipe 6, and the downstream end of the casing 8 isconnected to the upstream end of the second exhaust pipe 7. With thisstructure, it may be considered that the casing 8 is arranged in theexhaust pipes 6, 7. The first exhaust pipe 6, the casing 8, and thesecond exhaust pipe 7 are communicated with each other so that exhaustgas flows therethrough.

[0054] As shown in FIG. 1, the middle portion of the casing 8 has adiameter larger than that of the exhaust pipes 6, 7. Accordingly, theinterior of the casing 8 is larger than that of the exhaust pipes 6, 7.A ceramic filter assembly 9 is accommodated in the casing 8.

[0055] A thermal insulative material 10 is arranged between the outersurface of the assembly 9 and the inner surface of the casing 8. Thethermal insulative material 10 is a mat-like material including ceramicfibers and has a thickness of several millimeters to several tens ofmillimeters. It is preferred that the heat insulative material 10 bethermally expansive. Thermally expansive refers to the release ofthermal stress through an elastic structure. This is to minimize energyloss during reproduction by preventing heat from being released from theoutermost portion of the assembly 9. Further, the expansion of ceramicfibers using the heat produced during reproduction prevents displacementof the ceramic filter assembly 9, which would result from the pressureof the exhaust gas or vibrations produced by the moving vehicle.

[0056] The ceramic filter assembly 9 eliminates diesel particulates andit thus normally referred to as a diesel particulate filter (DPF). Asshown in FIG. 2 and FIG. 4, the assembly 9 is formed by bundling andintegrating a plurality of filters F. Elongated square filters F1 arearranged at the central portion of the assembly 9, and the outerdimension of the elongated square filter F1 is 33 mm×33 mm×167 mm (referto FIG. 3). Filters F1 that have forms differing from the elongatedsquare filters F1 are arranged about the elongated square filters F1.This forms the ceramic filter body 9, which as a whole, is cylindrical(diameter being about 135 mm).

[0057] These filters F1 are made of a sintered porous silicon carbide,which is one type of sintered ceramic. The reason for employing sinteredporous silicon carbide is because it is advantageous especially in thatit has superior heat resistance and heat conductance. In addition tosintered porous silicon carbide, the sintered material may be siliconnitride, sialon, alumina, cordierite, or mullite.

[0058] As shown in FIG. 3 and the other drawings, the filters F1 have ahoneycomb structure. The reason for employing the honeycomb structure isin that the pressure loss is small when the trapped amount of fineparticles increases. Each filter f1 has a plurality of through holes 12,which have generally square cross-sections and are arranged regularlyextending in the axial direction. The through holes 12 are partitionedfrom each other by thin cell walls 13. The outer surface of the cellwall 13 carries an oxide catalyst formed from a platinum group element(such as Pt) or other metal elements and there oxides. The opening ofeach through hole 12 on one of the end surfaces 9 a, 9 b is sealed by asealing body 14 (sintered porous silicon carbide body). Accordingly, theend surfaces 9 a, 9 b have a chessboard appearance. Thus, the filters F1have a plurality of cells having square cross-sections. The cell densityis about 200/inch, the thickness of the cell wall 13 is about 0.3 mm,and the cell pitch is about 1.8 mm. Among the plurality of cells, abouthalf are opened to the upstream end surface 9 a, and the others areopened at the downstream end surface 9 b.

[0059] The average porous diameter of the filter F1 is about 1 μm-50 μm,and more particularly, 51 μm-20 μm. If the average pore diameter is lessthan 1 μm, the deposited fine particles tend to clog the filter F1. Ifthe average pore diameter exceeds 50 μm, fine particles would not betrapped and would decrease the trapping efficiency.

[0060] It is preferred that the porosity rate be 30% to 70%, and moreparticularly, 40% to 60%. If the porosity rate is lower than 30%, thefilter F1 becomes too fine and may hinder the circulation of exhaust gastherein. If the porosity rate exceeds 70%, the amount of gaps in thefilters F1 becomes too large. This may decrease the strength of thefilters f1 and decrease the fine particle trapping efficiency.

[0061] When selecting the sintered porous silicon carbide, it ispreferred that the heat conductance of the filter F1 be 20 W/mK to 80W/mK, and more particularly, 30W/mK to 70 W/mK.

[0062] Referring to FIGS. 4 and 5, the outer surfaces of a total of 16filters F are adhered to one another by means of a ceramic seal layer15.

[0063] The ceramic seal layer 15 will now be described in detail.

[0064] It is preferred that the heat conductance of the seal layer 15 be0.1 W/mK-10 W/mK, and more particularly be 0.2 W/mK-2 W/mK.

[0065] If the heat conductance is less than 0.1 W/mK, the heatconductance of the seal layer 15 cannot be sufficiently improved. Thus,the seal layer 15 continues to be a large resistance and hinders heatconduction between filters F1. On the other hand, if the heatconductance exceeds 10 W/mK, properties such as adhesion and heatresistance may be degraded and cause manufacturing to-be difficult.

[0066] It is required that the thickness t1 of the seal layer 15 be 0.3mm-3 mm. Further, it is preferred that the thickness be 0.5 mm-2 mm.

[0067] If the thickness t1 exceeds 3 mm, the seal layer 15 continues tobe a large seal layer 15 even if the heat conductance is high and theheat conductance between the filters F1 is hindered. In addition, theratio of the assembly 9 occupied by the filters F1 would relativelydecrease and lower the filtration capacity. On the other hand, if thethickness t1 of the seal layer 15 is less than 0.3 mm, the seal layer 15would not become a large resistance However, the force adhering thefilters F1 to each other may become too low and cause the assembly 9 tobe vulnerable to breakage.

[0068] The seal layer 15 is formed from at least an inorganic fiber, aninorganic binder, an organic binder, and inorganic particles. Further,it is preferred that the seal layer 15 be an elastic material formed bybinding inorganic fibers and inorganic particles, whichthree-dimensionally intersect one another, with an inorganic binder andan organic binder.

[0069] At least one type of ceramic fiber selected from silica-aluminafiber, mullite fiber, alumina fiber, and silica fiber are selected asthe inorganic fiber included in the seal layer 15. Among these fibers,it is most preferred that silica-alumina ceramic fiber be selected.Silica-alumina ceramic fiber has superior elasticity and serves toabsorb thermal stress.

[0070] In this case, the content of the silica-alumina ceramic fiber inthe seal layer 15 is 10 wt %-70 wt %, preferably 10 wt %-40 wt %, andmore preferably 20 wt %-30 wt %. If the content is less than 10 wt-%,the thermal conductivity decreases and the elasticity decreases. If thecontent exceeds 70%, the thermal conductivity and elasticity decrease.

[0071] The shot content of the silica-alumina ceramic fiber is 1 wt %-10wt %, preferably 1 wt %-5 wt %, and more preferably 1 wt %-3 wt %. Ifthe shot content is less than 1 wt %, manufacture is difficult, and ifthe shot content is 50 wt %, the outer surface of the filter F1 may bedamaged.

[0072] The fiber length of silica-alumina ceramic fiber is 1 mm-10 mm,preferably 1 mm-50 mm, and more preferably 1 mm-20 mm. If the fiberlength is 1 mm or less, there is a disadvantage in that an elasticstructure cannot be formed. If the fiber length exceeds 100 mm, there isa disadvantage in that the fiber may produce balls of fibers anddecrease the dispersion of inorganic fine particles. Further, if thefiber length exceeds 100 mm, it becomes difficult to make the seal layerthinner than 3 mm and to improve the heat conductance between thefilters F1.

[0073] It is preferred that the inorganic binder included in the seallayer 15 be a colloidal sol selected from at least one of silica sol andalumina sol. It is especially preferred that silica sol be selected.This is because silica sol is optimal for use as an adhesive agent underhigh temperatures since it is easily obtained easily sintered to SiO₂.In addition, silica sol has a superior insulative characteristic.

[0074] In this case, the content of silica sol in the seal layer 15 as asolid is 1 wt %-30 wt %, preferably 1 wt %-15 wt %, and more preferably5 wt %-9 wt %. If the content is less than 1 wt %, the adhesive strengthdecreases. On the other hand, if the content exceeds 30 wt %, thethermal conductivity decreases.

[0075] It is preferred that the organic binder included in the seallayer 15 be a hydrophilic organic high polymer and also be preferredthat the organic binder be a polysaccharide selected from at least oneof poly vinyl alcohol, methyl cellulose, ethyl cellulose, andcarboxymethyl cellulose. It is especially preferred that carboxymethylcellulose be selected. This is because the seal layer 15 has optimalfluidity due to carboxymethyl cellulose and thus has superior adhesionunder normal temperatures.

[0076] In this case, the content of carboxymethyl cellulose as a solidis 0.1 wt %-5.0 wt %, preferably 0.2 wt %-1.0 wt %, and more preferably0.4 wt %-0.6 wt %. If the content is less than 0.1 wt %, sufficientinhibition of migration becomes difficult. Migration refers to aphenomenon in which the binder in the seal layer 15 moves as the solventis removed as it dries when the seal layer 15 charged between the sealedbodies hardens. If the content exceeds 5.0 wt %, high temperature burnsand eliminates the organic binder and decreases the strength of the seallayer 15.

[0077] It is preferred that the inorganic particles included in the seallayer 15 be an inorganic powder or an elastic material employing awhisker that is selected from at least one of silicon carbide, siliconnitride, and boron nitride. Such carbide and nitrides have an extremelyhigh thermal conductivity and, when included in the surface of a ceramicfiber or in the surface of inside a colloidal sol, contributes toincreasing the thermal conductivity.

[0078] Among the above carbide and nitrides, it is especially preferredthat the silicon carbide powder be selected. This is because the thermalconductivity of silicon carbide is extremely high and easily adapts toceramic fiber. In addition, in the first embodiment, the filter F1,which is the sealed body, is made of sintered porous silicon carbide.Thus, it is preferred that the same type of silicon carbide powder beselected.

[0079] In this case, it is preferred that the content of the siliconcarbide powder as a solid be 3 wt %-80 wt %, preferably 10 wt %-60 wt %,and more particularly, 20 wt %-40 wt %. If the content is 3 wt % orless, the thermal conductivity of the seal layer 15 decreases andresults in the seal layer 15 having a large heat resistance. If thecontent exceeds 80 wt %, the adhesion strength decreases when thetemperature is high.

[0080] The grain diameter is 0.01 μm-100 μm, preferably 0.1 μm-15 μm,and more preferably 0.1 μm-10 μm. If the grain diameter exceeds 100 μm,the adhesion and thermal conductivity decrease. If the grain diameter isless than 0.01 μm, the cost of the seal material 15 increases.

[0081] The procedure for manufacturing the ceramic filter assembly 9will now be discussed.

[0082] First, a ceramic raw material slurry used during an extrusionprocess, a sealing paste used during an end surface sealing process, anda seal layer formation paste used during a filter adhesion process areprepared.

[0083] The ceramic raw material slurry is prepared by combining andkneading predetermined amounts of an organic binder and water withsilicon carbide particles. The sealing paste is prepared by combiningand kneading an organic binder, a lubricative agent, a plastic agent,and water with silicon carbide powder. The seal layer formation paste isprepared by combining and kneading predetermined amounts of an inorganicfiber, an inorganic binder, an organic binder, and inorganic particles,and water.

[0084] Next, the ceramic raw material slurry is put into an extruder andcontinuously extruded from a mold. Afterward, the extruded honeycombmolded product is cut into equivalent lengths to obtain elongated squarehoneycomb molded product pieces. Further, a predetermined amount ofsealing paste is charged into one of the openings of each cell in thecut pieces such that both end surfaces of each cut piece is sealed.

[0085] Then, main sintering is performed by setting predeterminedconditions, such as the temperature and time, to completely sinter thehoneycomb molded pieces and the sealing bodies 14. All of the sinteredporous silicon carbide filters F1 obtained in this manner are stillsquare pole-shaped.

[0086] The sintering temperature is set to 2,100° C. to 2,300° C. in thepresent embodiment to obtain the average pore diameter of 6 μm-15 μm anda porosity of 35% to 50%. Further, the sintering time is set to 0.1hours to 5 hours. Further, the interior of a furnace has an inertatmosphere during sintering, and the pressure in that atmosphere is thenormal pressure.

[0087] Then, after forming a ceramic bedding layer to the outer surfaceof the filters F1 as required, the seal layer formation paste is appliedthereto. The outer surfaces of sixteen of such filters F1 are adhered toeach other and thus integrated.

[0088] In the following outer form cutting process, the assembly 9,which has been obtained through the filter adherence process and has asquare cross-section, is ground to form the outer shape of the assembly9 by eliminating unnecessary sections from the peripheral portion of theassembly 9 and form the ceramic filter assembly 9, which cross-sectionis round.

[0089] The fine particle trapping performed by the ceramic filterassembly 9 will now be described briefly.

[0090] The ceramic filter assembly 9 accommodated in the casing 9 a issupplied with exhaust gas. The exhaust gas supplied via the firstexhaust pipe 6 first enters the cells that are opened at the upstreamend surface 9 a. The exhaust gas than passes through the cell wall 13and enters the adjacent cells, or the cells that are opened at thedownstream end surface 9 b. From the openings of these cells, theexhaust gas flows cut of the downstream end surfaces 9 b of the filtersF1. However, the fine particles included in the exhaust gas do not passthrough the cell walls 13 and are trapped by the cell walls 13. As aresult, the purified gas is discharged from the downstream end surface 9b of the filters F1. The purified exhaust gas then passes through thesecond exhaust pipe 7 to be ultimately discharged into the atmosphere.The trapped fine particles are ignited and burned by the catalyticeffect that occurs when the internal temperature of the assembly 9reaches a predetermined temperature.

EXAMPLE 1-1

[0091] (1) 51.5 wt % of a silicon carbide powder having an average graindiameter of 10 μm and 22 wt % of α silicon carbide powder having anaverage grain diameter of 0.5 μm were wet-mixed. Then, 6.5 wt % of theorganic binder (methyl cellulose) and 20 wt % of water were added to theobtained mixture and kneaded. Next, a small amount of the plastic agentand the lubricative agent were added to the kneaded mixture, furtherkneaded, and extruded to obtain the honeycomb molded product. Morespecifically, the a silicon carbide powder having an average particlediameter of about 10 μm was produced by Yakushima Denkou KabushikiKaisha under the product name of C-1000F, and the α silicon carbidepowder having an average particle diameter of about 0.5 μm was producedby Yakushima Denkou Kabushiki Kaisha under the product name of GC-15.

[0092] (2) Then, after drying the molded product with a microwave dryer,the through holes 12 of the molded product was sealed by the sealingpaste made of sintered porous silicon carbide. Afterward, the sealingpaste was dried again with the dryer. After the end surface sealingprocess, the dried body was degreased at 400° C. and then sintered forabout three hours at 2,200° C. in an argon atmosphere at the normalpressure. This obtained the porous, honeycomb, silicon carbide filtersF1.

[0093] (3) 23.3 wt % of a ceramic fiber (alumina silicate ceramic fiber,shot content 3%, fiber length 0.1 mm-100 mm), 30.2 wt % of siliconcarbide having an average grain diameter of 0.3 μm, 7 wt % of silica sol(the converted amount of SiO₂ of the sol being 30%) serving as theinorganic binder, 0.5 wt % of carboxymethyl cellulose serving as theorganic binder, and 39 wt % of water were mixed and kneaded. The kneadedmaterial was adjusted to an-appropriate viscosity to prepare the pasteused to form the seal layer 15.

[0094] (4) Then, the seal layer forming paste was uniformly applied tothe outer surface of the filters F1. Further, in a state in which theouter surfaces of the filters F1 were adhered to one another, thefilters F1 were dried and hardened under the condition of 50° C. to 100°C.×1 hour. As a result, the seal layer 15 adhered the filters F1 to oneanother. The thickness t1 of the seal layer 15 was set at 0.5 mm. Theheat conductivity of the seal layer 15 was 0.3 W/mK.

[0095] (5) Next, the peripheral portion was cut to shape the peripheralportion and complete the ceramic filter assembly 9, which has a roundcross-section.

[0096] Then, the thermal insulative material 10 is wound about theassembly 9 obtained in the above manner. In this state, the assembly 9is accommodated in the casing 8 and actually supplied with exhaust gas.After a predetermined time elapses, the assembly 9 is removed and cut ata plurality of locations. The cut surfaces were observed with the nakedeye.

[0097] Consequently, residuals of the fine particles were not confirmedat the peripheral portion of the assembly 9 (especially, the peripheralportion near the downstream end surface) where there is a tendency forunburned particles to remain. The fine particles were of coursecompletely burned at other portions. It is considered that such resultsare obtained because the usage of the seal layer 15 prevents theconductance of heat between the filters F1 from being decreased and thetemperature sufficiently increases at the peripheral portion of theassembly 9. Accordingly, in example 1-1, it is apparent that exhaust gaswas efficiently processed.

EXAMPLES 1-2, 1-3

[0098] In example 1-2, the ceramic filter assembly 9 was prepared bysetting the thickness t1 of the seal layer 15 at 1.0 mm. The otherconditions were basically set in accordance with example 1-1. In example3, the ceramic filter assembly 9 was formed by setting the thickness t1of the seal layer 15 at 2.5 mm. The other conditions were basically setin accordance with example 1-1.

[0099] Then, the obtained two types of assemblies 9 were used for acertain period, and the cut surfaces were observed with the naked eye.The same desirable results as example 1-1 were obtained. Thus, it isapparent that the exhaust gas was efficiently processed in examples 1-2and 1-3.

EXAMPLE 1-4

[0100] In example 1-4, the employed seal layer forming paste wasprepared by mixing and kneading 25 wt % of a ceramic fiber (mullitefiber, shot content rate 5 wt %, fiber length 0.1 mm-100 mm), 30 wt % ofsilicon nitride powder having an average grain diameter of 1.0 μm, 7 wt% of alumina sol (the conversion amount of alumina sol being 20% servingas an inorganic binder, 0.5 wt % of poly vinyl alcohol serving as anorganic binder, and 37.5 wt % of alcohol. The other portions were formedin accordance with example 1-1 to complete the ceramic filter assembly9. The thickness t1 of the seal layer 15 was set at 1.0 mm. The thermalconductivity of the seal layer 15 was 0.2 W/mK.

[0101] Then, the obtained assembly 9 was used for a certain period, andthe cut surfaces were observed with the naked eye. The same desirableresults as example 1 were obtained. Thus, it is apparent that theexhaust gas was efficiently processed in example 4.

EXAMPLE 1-5

[0102] In example 1-5, the employed seal layer forming paste wasprepared by mixing and kneading 23 wt % of a ceramic fiber (aluminafiber, shot content rate 4 wt %, fiber length 0.1 mm-100 mm), 35 wt % ofboron nitride powder having an average grain diameter of 1 μm, 8 wt % ofalumina sol (the conversion amount of alumina sol being 20%) serving asan inorganic binder, 0.5 wt % of ethyl cellulose serving as an organicbinder, and 35.5 wt % of acetone. The other portions were formed inaccordance with example 1 to complete the ceramic filter assembly 9. Thethickness t1 of the seal layer 15 was set at 11.0 mm. The thermalconductivity of the seal layer 15 was 2 W/mK.

[0103] Then, the obtained assembly 9 was used for a certain period, andthe cut surfaces were observed with the naked eye. The same desirableresults as example 1 were obtained. Thus, it is apparent that theexhaust gas was efficiently processed in example 5.

[0104] The ceramic filter assembly 9 of the first embodiment has thefollowing advantages:

[0105] (1) In each example, the thickness t1 of the seal layer 15 is setin the preferable range of 0.3 mm-3 mm, and the thermal conductivity ofthe seal layer 15 is set in the preferable range of 0.1 W/mK-10 W/mK.This improves the thermal conductivity of the seal layer and preventsthe thermal conductivity between the filters F1 from being decreased.Accordingly, heat is uniformly and quickly conducted to the entireassembly 9. This prevents a temperature difference from being producedin the assembly 9. Accordingly, the thermal uniformity of the assembly 9is increased and the occurrence of locally unburned particles isavoided. The exhaust gas purification apparatus 1, which uses theassembly 9, has superior exhaust gas processing efficiency.

[0106] Further, if the thickness t1 and the thermal conductivity iswithin the above range, basic properties, such as adhesiveness and heatresistance remain the same. This avoids the manufacturing of the seallayer 15 from becoming difficult. Further, since the seal layer 15serves to adhere the filters F1 to one another, breakage of the assembly9 is avoided. In other words, the assembly 9 is relatively easy tomanufacture and has superior durability.

[0107] (2) The seal layer 15 in each example contains as a solid 10 wt%-70 wt % of ceramic fibers. This enables the seal layer 15 to have highthermal conductivity and elasticity. Thus, the thermal conductivitybetween filters F1 is improved, and the thermal conductivity of theassembly 9 is further increased.

[0108] (3) The seal layer 15 in each example contains ceramic fibers,the lengths of which are 100 mm or shorter. Accordingly, the thicknesst1 of the seal layer 15 may be set to 3 mm or less without anydifficulties. This increases the heat conductivity between the filtersF1, and thus contributes to the thermal uniformity of the assembly 9.

[0109] (4) The seal layer 15 in each example contains as a solid 3 wt%-80 wt % of inorganic particles. Thus, the seal layer 15 has highthermal conductivity. This increases the heat conductivity between thefilters F1 and contributes to the thermal uniformity of the assembly 9.

[0110] (5) The seal layer 15 in the above examples are formed from atleast an inorganic fiber, an inorganic binder, an organic binder, andinorganic particles. Further, the seal layer 15 is made of an elasticmaterial formed by joining three-dimensionally intersecting theinorganic fibers with the inorganic particles with an inorganic binderand an organic binder.

[0111] Such material has the advantages described below. Sufficientadhesion strength is obtained in a low temperature range and a hightemperature range. Further, the material is elastic. Thus, when thermalstress is applied to the assembly 9, the release of the thermal stressis ensured.

[0112] The first embodiment of the present invention may be modified asdescribed below.

[0113] (a) The number of the filters F1 is not limited to 16 and may beany number. In this case, filters F1 having different dimensions andshapes may be combined.

[0114] (b) With reference to FIG. 6, in a ceramic filter assembly 21 ofa further embodiment, the filters F1 are offset from one another in adirection perpendicular to the filter axial direction, and the filtersF1 are adhered by the seal layer 15. In this case, the filters F1resists displacement when being accommodated in the casing 8. Thisimproves the breakage strength of the assembly 21. In the ceramic filterassembly 21 of FIG. 6, the seal layer 15 does not include cross-likeportions. It is considered that this contributes to improvement of thebreakage strength. Further, since the thermal conductivity in the radialdirection of the assembly 21 is further improved, the thermal uniformityof the assembly 21 is further enhanced.

[0115] (c) Instead of the honeycomb filters F1, the filters may have athree-dimensional mesh structure, a foam-like structure, a noodle-likestructure, or a fiber-like structure.

[0116] (d) Prior to the outer form cutting process, the form of thefilter F1 is not limited to the elongated square shape and may have atriangular pole-like shape or a hexagonal pole-like shape. Further, theassembly 9 does not necessarily have to be formed to have a roundcross-section during the outer form cutting process and may be formed tohave a, for example, oval cross-section.

[0117]FIG. 7 is a perspective view showing a honeycomb filter F10 of aceramic filter assembly in a second embodiment of the present invention.FIG. 8 is an enlarged cross-sectional view showing the main portion ofthe exhaust gas purification apparatus. The corners on'the outer surfaceof the honeycomb filter F10 are curved to define round surfaces 18. Itis required that the curvature of the round surfaces 18 be R=0.3 to 2.5.It is further preferred that the curvature be R=0.7 to. 2.5, andparticularly preferred that the curvature be R=1.0 to 2.0.

[0118] When the curvature R is 0.3 or less, the corners are stillangulated. Thus, the concentration of stress to the corners cannot besufficiently avoided and the corners may chip or crack. On the otherhand, if the curvature R exceeds 2.5, the cross-sectional area of thehoneycomb filter F1 decreases. This reduces the effective number ofcells and decreases the filtering capability of the assembly 29.

[0119] The ceramic filter assembly of the second embodiment ismanufactured by chamfering each corner of an elongated square honeycombmolded product piece and forming the round surfaces 18 with thepredetermined curvature R.

EXAMPLE 2-1

[0120] In example 2-1, the ceramic filter assembly 29 was manufacturedby drying molded products with a microwave dryer, cutting off eachcorner to perform chamfering, and forming the round surfaces 18 ofR=1.5. The other steps are in accordance with example 1-1.

[0121] An assembly 29 obtained in the above manner was actually suppliedwith exhaust gas. After a predetermined time, the assembly 29 wasremoved and observed with the naked eye.

[0122] The result revealed that there were no cracks originating fromthe corners in the seal layer 15. Further, there was no chipping of thecorners. Accordingly, it has become apparent that the assembly 29 of theexample 2-1 is extremely superior in strength.

EXAMPLES 2-2, 2-3

[0123] In example 2, the ceramic filter assembly 9 was manufactured bysetting the curvature of the round surfaces 18 at R=0.4 and forming theother portions basically in the same manner as in example 2-1. Inexample 2-3, the ceramic filter assembly 29 was manufactured by settingthe curvature of the round surfaces 18 at R=2.4 and forming the otherportions basically in the same manner as in example 2-1.

[0124] Then, the obtained two types of the assembly 29 were used for acertain time period in the same manner as example 2-1 and observed withthe naked eye. A preferable result similar to that of example 2-1 wasobtained. In other words, it has become apparent that the assemblies 29of the examples 2-2, 2-3 are superior in strength.

EXAMPLE 2-4

[0125] In example 2-4, the ceramic filter assembly 29 was manufacturedby using a seal layer forming paste in the same manner as in example 1-4and forming the other portions in the same manner as in example 2-1. Thethickness of the seal layer was set at 1.0 mm, and the curvature of theround surface 18 of each corner was set at R=1.5.

[0126] Then, the obtained assembly 29 was used for a certain time periodin the same manner as example 2-1 and observed with the naked eye. Apreferable result similar to that of example 2-1 was obtained. In otherwords, it has become apparent that the assembly 29 of example 2-4 issuperior in strength.

EXAMPLE 2-5

[0127] In example 2-5, the ceramic filter assembly 29 was manufacturedby using a seal layer forming paste in the same manner as in example 1-5and forming the other portions in the same manner as in example 2-1. Thethickness of the seal layer was set at 1.0 mm, and the curvature of theround surface 18 of each corner was set at R=1.5.

[0128] Then, the obtained assembly 29 was used for a certain time periodin the same manner as example 2-1 and observed with the naked eye. Apreferable result similar to that of example 2-1 was obtained.

COMPARATIVE EXAMPLE

[0129] In the comparative example, the ceramic filter assembly 9 wasmanufactured without chamfering the corners and forming the otherportions in the same manner as in example 2-1. Thus, the honeycombfilters F1 of the assembly 29 have angulated corners.

[0130] Then, the obtained assembly 29 was used for a certain time periodin the same manner as example 2-1 and observed with the naked eye.Cracks and chipping caused by stress concentration were discovered atmultiple locations. Accordingly, the assembly 29 was inferior instrength.

[0131] The ceramic filter assembly of the second embodiment has theadvantages discussed below.

[0132] (1) The corners on the outer surface of the honeycomb filter F1are round surfaces 18 having a curvature in an optimal range. Thisavoids stress concentration at the corners. Accordingly, the chipping ofthe corners of the honeycomb filter F1, the cracking of the seal layer15 from the corners is prevented, and the ceramic filter assembly 29resists breakage. This increases the strength of the assembly 29 andimproves the strength and filtering capability of the exhaust gaspurification apparatus 1, which employs the assembly 29.

[0133] (2) The assembly 29 employs the honeycomb filter 1, which is madeof honeycomb sintered porous silicon carbide. As a result, the obtainedassembly 29 has a higher filtering capability, less pressure loss, andsuperior heat resistance and heat conductance characteristics.

[0134] The second embodiment may be modified as described below.

[0135] With reference to FIG. 9, the present invention may be embodiedin a ceramic filter assembly 221 by offsetting the filters F1 from oneanother in a direction perpendicular to the filter axial direction.

[0136] Instead of forming the round surfaces 18 by chamfering thecorners, the round surfaces may be formed simultaneously when moldingthe honeycomb molded product with a mold.

[0137] The honeycomb filter F1 is not required to be shaped into arectangular pole, which has a square cross-section, prior to the outerform cutting process. For example, as shown in FIG. 10, a honeycombfilter F20 may be formed into a rectangular pole having a rectangularcross-section. Further, a honeycomb filter F30 may be triangular asshown in FIG. 11, and a honeycomb filter F40 may be hexagonal as shownin FIG. 12.

[0138]FIG. 13 is a schematic cross-sectional view showing a ceramicfilter 39 according to a third embodiment of the present invention.

[0139] Referring to FIG. 13 and FIG. 14(b), the ceramic filter assembly39 of the third embodiment has an outer surface 39 c to which a ceramicsmoothing layer 16 is applied. The smoothing layer 16 is made of aceramic material that includes at least ceramic fibers and a binder. Itis preferred that the ceramic material includes inorganic particles,such as silicon carbide, silicon nitride, and boron nitride. It ispreferred that an inorganic binder, such as silica sol or alumina sol,or an organic binder, such as a polysaccharide, be used as the binder.It is preferred that the ceramic material be formed by bindingthree-dimensionally intersecting ceramic fibers and inorganic particleswith a binder. It is preferred that the smoothing layer 16 be formedfrom the same type of material as the seal layer 15, and especiallypreferred that the smoothing layer 16 be made of exactly the samematerial as the seal layer 15.

[0140] It is preferred that the thickness of the smoothing layer 16 be0.1 mm to 10 mm, further preferred that the thickness be 0.3 mm to 2 mm,and optimal that the thickness be 0.5 mm to 1 mm. If the smoothing layer16 is too thin, pits 17 that are formed in the outer surface 9 c of theceramic filter assembly 9 cannot be completely filled. Thus, gaps tendto remain in such locations. On the other hand, if the smoothing layer16 is thickened, the formation of the layer may become difficult, andthe diameter of the entire assembly 9 may be enlarged.

[0141] It is preferred that the seal layer 15 be formed thinner than thesmoothing layer 16, and more particularly, be formed in the range of 0.3mm to 3 mm. When the seal layer 15 is thinner than the smoothing layer,the filtering capacity and the thermal conductance are prevented frombeing decreased beforehand.

[0142] The procedure for manufacturing the ceramic filter assembly 39will now be described with reference to FIG. 14.

[0143] First, a ceramic raw material slurry used during an extrusionprocess, a sealing paste used during an end surface sealing process, aseal layer formation paste used during a filter adhesion process, and asmoothing layer formation paste are prepared. When using the seal layerformation paste to form the smoothing layer, the smoothing layerformation paste does not have to be prepared.

[0144] The ceramic raw material slurry is prepared by combining andkneading predetermined amounts of an organic binder and water withsilicon carbide particles. The sealing paste is prepared by combiningand kneading an inorganic binder, a lubricative agent, a plastic agent,and water with silicon carbide powder. The seal layer formation paste(smoothing layer formation paste) is prepared by combining and kneadingpredetermined amounts of an inorganic fiber, an inorganic binder, anorganic binder, inorganic particles, and water.

[0145] Next, the ceramic raw material slurry is put into an extruder andcontinuously extruded from a mold. Afterward, the extruded honeycombmolded product is cut into equivalent lengths to obtain elongated squarehoneycomb molded product pieces. Further, a predetermined amount of thesealing paste is charged into one of the openings of each cell in thecut pieces to seal both end surfaces of each cut piece.

[0146] Then, main sintering is performed by setting predeterminedconditions, such as the temperature and time, to completely sinter thehoneycomb molded pieces and the sealing bodies 14. All of the sinteredporous silicon carbide filters F1 obtained in this manner are stillsquare pole-shaped.

[0147] The sintering temperature is set to 2,100° C. to 2,300° C. in thepresent embodiment to obtain the average pore diameter of 6 μm to 15 μmand a porosity of 35% to 50%. Further, the sintering time is set to 0.1hours to 5 hours Further, the interior of a furnace has an inertatmosphere during sintering, and the pressure in that atmosphere is thenormal pressure.

[0148] Then, after forming a ceramic bedding layer to the outer surfaceof the filters F1 as required, the seal layer formation paste is appliedthereto. The outer surfaces of sixteen of such filters F1 are adhered toeach other and thus integrated. At this point, the ceramic filterassembly 39A as a whole has a square cross-section, as shown in FIG.14(a).

[0149] In the following outer form cutting process, the assembly 39A,which has been obtained through the filter adherence process and has asquare cross-section, is ground to form the outer shape of the assembly9 by eliminating unnecessary sections from the peripheral portion of theassembly 39A.

[0150] As a result, the ceramic filter assembly 39 having aroundcross-section is obtained, as shown in FIG. 14(b) Cell walls 13 arepartially exposed from the surface formed during the outer form cutting.Thus, pits 17 are formed in the outer surface 39 c. The pits 17 areabout 0.5 mm to 1 mm and are defined by ridges and valleys extending inthe axial direction of the assembly 39 (i.e., the longitudinal directionof the filters F1).

[0151] In the following smoothing layer forming process, the seal layerformation paste is used as the smoothing layer formation paste anduniformly applied to the outer surface 9 c of the assembly 39. Thiscompletes the ceramic filter assembly 39 shown in FIG. 14(c).

EXAMPLE 3-1

[0152] (1) 51.5 wt % of a silicon carbide powder and 22 wt % of βsilicon carbide powder were wet-mixed. Then, 6.5 wt % of the organicbinder (methyl cellulose) and 20 wt % of water were added to theobtained mixture and kneaded. Next, a small amount of the plastic agentand the lubricative agent were added to the kneaded mixture, furtherkneaded, and extruded to obtain the honeycomb molded product.

[0153] (2) Then, after drying the molded product with a microwave dryer,the through holes 12 of the molded product were sealed by the sealingpaste made of sintered porous silicon carbide. Afterward, the sealingpaste was dried again with the dryer. After the end surface sealingprocess, the dried body was degreased at 400° C. and then sintered forabout three hours at 2,200° C. in an argon atmosphere at normalpressure. This obtained the porous, honeycomb, silicon carbide filtersF1.

[0154] (3) 23.3 wt % of a ceramic fiber (alumina silicate ceramic fiber,shot content 3%, fiber length 0.1 mm-100 mm), 30.2 wt % of siliconcarbide having an average grain diameter of 0.3 μm, 7 wt % of silica sol(the converted amount of SiO₂ of the sol being 30%) serving as theinorganic binder, 0.5 wt % of carboxymethyl cellulose serving as theorganic binder, and 39 wt % of water were mixed and kneaded. The kneadedmaterial was adjusted to an appropriate viscosity to prepare the pasteused to form the seal layer 15 and the smoothing layer 16.

[0155] (4) Then, the seal layer forming paste was uniformly applied tothe outer surface of the filters F1. Further, in a state in which theouter surfaces of the filters F1 were adhered to one another, thefilters F1 were dried and hardened under the condition of 50° C. to 100°C.×1 hour. As a result, the seal layer 15 adhered the filters F1 to oneanother. The thickness t1 of the seal layer 15 was set at 1.0 mm.

[0156] (5) Next, the peripheral portion was cut to shape the peripheralportion and complete the ceramic filter assembly 39, which has a roundcross-section. Then, the seal and smoothing paste was uniformly appliedto the expose outer surface 39 c. The smoothing layer 16 having athickness of 0.6 mm was dried and hardened under the condition of 50° C.to 100° C.×1 hour to complete the assembly 39.

[0157] The assembly 39 obtained in the above manner was observed withthe naked eye. The pits 17 in the outer surface 39 c were substantiallycompletely filled by the smoothing layer 16, and the outer surface 39 cwas smooth. Further, there were no cracks in the boundary portions ofthe smoothing layer 16 and the seal layer 15. Accordingly, thisindicates that the levels of adhesion and seal were high at the boundaryportions.

[0158] No gaps were formed in the outer surface 9 c of the assembly 39when accommodating the assembly 39 encompassed by the thermal insulativematerial in the casing 8. Further, when actually supplying exhaust gas,there was no leakage of the exhaust gas through the gaps in the outersurface 39 c from the downstream side. It is thus apparent that exhaustgas is efficiently processed in the third embodiment.

EXAMPLE 3-2

[0159] In example 3-2, the seal and smoothing paste was prepared bymixing and kneading 25 wt % of a ceramic fiber (mullite fiber, shotcontent rate 5 wt %, fiber length 0.1 mm-100 mm), 30 wt % of siliconnitride powder having an average grain diameter of 1.0 μm, 7 wt % ofalumina sol (the conversion amount of alumina sol being 20%) serving asan inorganic binder, 0.5 wt % of poly vinyl alcohol serving as anorganic binder, and 37.5 wt % of alcohol. The other portions were formedin accordance with example 3-1 to complete the ceramic filter assembly39.

[0160] Then, observations were made by the naked eye in the same manneras example 1. The pits 17 in the outer surface 39 c were substantiallycompletely filled by the smoothing layer 16. Further, there were nocracks in the boundary portions of the smoothing layer 16 and the seallayer 15. Accordingly, this indicates that the levels of adhesion andseal were high at the boundary portions.

[0161] No gaps were formed in the outer surface 39 c of the assembly 39during usage. In addition, leakage of exhaust gas through gaps in theouter surface 39 c did not occur. It is thus apparent that exhaust gaswas efficiently processed in example 3-2 in the same manner as example3-1.

EXAMPLE 3-3

[0162] In example 3-3, the seal and smoothing paste was prepared bymixing and kneading 23 wt % of a ceramic fiber (alumina fiber, shotcontent rate 4 wt %, fiber length 0.1 mm-100 mm), 35 wt % of boronnitride powder having an average grain diameter of lam, 8 wt % ofalumina sol (the conversion amount of alumina sol being 20%) serving asthe inorganic binder, 0.5 wt % of ethyl cellulose serving as the organicbinder, and 35.5 wt % of acetone. The other portions were formed inaccordance with example 3-1 to complete the ceramic filter assembly 39.

[0163] Then, observations were made by the naked eye in the same manneras example 0.3-1. The pits 17 in the outer surface 39 c weresubstantially completely filled by the smoothing layer 16. Further,there were no cracks in the boundary portions of the smoothing layer 16and the seal layer 15. Accordingly, this indicates that the levels ofadhesion and seal were high at the boundary portions.

[0164] No gaps were formed in the outer surface 39 c of the assembly 39during usage. In addition, leakage of exhaust gas through gaps in theouter surface 39 c did not occur. It is thus apparent that exhaust gaswas efficiently processed in example 3-3 in the same manner as example3-1.

COMPARATIVE EXAMPLE

[0165] In the comparative example, the smoothing layer 16 was not formedon the outer surface 39 c. The other portions were formed in accordancewith example 3-1 to complete a ceramic filter assembly.

[0166] Then, observations were made by the naked eye in the same manneras example 3-1. There were pits 17 in the outer surface 3-9 c. Thus,gaps were formed in the outer surface 3-9 c during usage of theassembly, and gas leakage through the gaps occurred. Accordingly, incomparison with examples 3-1 to 3-3, it is apparent that the exhaust gasprocessing efficiency was inferior.

[0167] Accordingly, the ceramic filter assembly 39 has the advantagesdescribed below.

[0168] (1) The smoothing layer 16 fills the pits 17 and smoothes theouter surface 9 c. Accordingly, gaps are not formed in the outer surface39 c when the assembly 39 is retained. This prevents the leakage ofexhaust gas. As a result, the ceramic filter assembly 39 has superiorexhaust gas processing efficiency. This, in turn, results in the exhaustgas purification apparatus 1 having superior exhaust gas processingefficiency.

[0169] The smoothing layer 16 is made of ceramic and thus has superioradhesion with the filter F1, which is made of a sintered porous ceramic,and superior heat resistance. Accordingly, even if the assembly 39 isexposed to a high temperature of several hundred degrees Celsius, thesmoothing layer 16 is not burned nor deformed. Thus, the desiredadhesion strength is maintained.

[0170] (2) The thickness of the smoothing layer 16 is set in thepreferred range of 0.1 mm to 10 mm. This prevents the leakage of exhaustgas without making the manufacture of the assembly 39 difficult.

[0171] (3) The seal layer 15 is thinner than the smoothing layer 16.This prevents the filtering capability and the thermal conductivity fromdecreasing.

[0172] (4) The smoothing layer 16 is made from the same material as theseal layer 15. Since the coefficient of thermal expansion of thesmoothing layer 16 and that of the seal layer 15 are the same, theboundary portions of the seal and smoothing layer 15, 16 do not crack.In other words, high adhesiveness, sealing, and reliability of theboundary portions are ensured.

[0173] Further, a smoothing layer formation paste does not have to beprepared in addition to the seal layer formation paste. This facilitatesthe manufacture of the assembly 39 and avoids an increase in themanufacturing cost.

[0174] (5) The following may be used as the material for forming theseal layer 15 and the smoothing layer 16. An elastic material includingat least an inorganic fiber, an inorganic binder, an organic binder, andinorganic particles and bound to one another by the inorganic binder andthe organic binder may be used.

[0175] Such material has the advantage described below. The material hassatisfactory adhesion strength in both low temperature and hightemperature ranges. Further, the material is an elastic material. Thus,when thermal stress is applied, the thermal stress is relieved. Further,the material has superior thermal conductance. Thus, heat is uniformlyand quickly conducted to the entire assembly 39. This enables efficientexhaust gas processing.

[0176] The third embodiment of the present invention may be modified asdescribed below.

[0177] (a) As shown in FIG. 15, the present invention may be embodied ina ceramic filter assembly 321 by offsetting the filters F1 from oneanother in a direction perpendicular to the filter axial direction.

[0178] (b) The smoothing layer 16 may be formed from a ceramic materialthat differs from that of the seal layer 15.

[0179] (c) The smoothing layer 16 may have the same thickness as theseal layer 15 or may have a greater thickness than the seal layer 15.

[0180] (d) In addition to forming the smoothing layer 16 by employing anapplication technique, other methods, such as a printing technique, astaining technique, a dip technique, and a curtain coat technique, maybe employed.

[0181]FIG. 16 is a schematic perspective view of a ceramic filterassembly 49 according to a fourth embodiment of the present invention.The ceramic filter assembly 49 is formed by a plurality of rectangularpole-like honeycomb filters F100.

[0182] In each honeycomb filter F100, the flow direction of the exhaustgas (direction perpendicular to the filter end surface), which is theprocessed fluid, is defined as the filter length L (mm). Further, thearea obtained when cutting each honeycomb filter F100 in a directionperpendicular to the flow direction (in other words, parallel to thefilter end surface) is defined as the filter cross-sectional area S(mm²).

[0183] In this case, the L/S value must be 0.06 mm/mm² to 0.75 mm/mm².It is preferred that the L/S value be 0.10 mm/mm² to 0.60 mm/mm², andmost preferred that the L/S value be 0.15 mm/mm² to 0.40 mm/mm².

[0184] When the L/S value exceeds 0.75 mm/mm², a temperature differenceis produced in the longitudinal direction of the filter. As a result, ahigh level of thermal stress is applied to the honeycomb filter F100thereby permitting cracks to easily form. On the other hand, when theL/S value is 0.06 mm/mm² or less, a temperature difference is producedin a direction perpendicular to the filter longitudinal direction. Thisalso applies a high level of thermal stress to the honeycomb filter F100thereby permitting cracks to easily form.

[0185] It is specifically preferred that the filter length L be 120 mmto 300 mm, and especially preferred that the filter length be 140 mm to200 mm. It is specifically preferred that the filter cross-sectionalarea S be 400 mm² to 2,500 mm², and especially preferred that thecross-sectional area S be 600 mm² to 2,000 mm², and especially preferredthat the cross-sectional area S be 600 mm² to 2,000 mm². When the valuesof L and S are outside the preferred range, a temperature difference isproduced in the honeycomb filter F100. As a result, a large level ofthermal stress easily forms.

EXAMPLE 4-1

[0186] Basically, the same assembly 49 as that of example 1-1 wasmanufactured. The height W of the filter F100 was 33 mm, the width W2was 33 mm, and the length L was 167 mm. Accordingly, the filtercross-sectional area S was 1,089 mm², and the L/S value was 0.15 mm/mm²(=167/1089).

[0187] Then, the thermal insulative material 10 was wrapped around theassembly 49. In this state, the assembly was retained in the casing 8and actually supplied with exhaust gas.

[0188] Referring to FIGS. 18(A) and 18(B), thermocouples were embeddedin each of locations P1 to P6 and temperatures T1 to T6 wererespectively measured for a certain period, respectively. Further,maximum temperature differences A T(° C.) at each of the locations P1 toP6 were obtained. The white arrow in the drawing shows the direction ofthe exhaust gas flow. The temperature measurement was conducted on thehoneycomb filter F100 denoted with reference character X in FIG. 16.

[0189] After a predetermined time, the assembly 49 was taken out and thehoneycomb filters. F100 were each observed with the naked eye. As aresult, the maximum temperature difference ΔT(° C.) of example 4-1 wasabout 5° C., the value of which is extremely small. Further, cracks werenot confirmed in any of the honeycomb filters F100.

EXAMPLES 4-2 TO 4-6

[0190] In examples 4 to 6, the assembly 49 was manufactured in the samemanner as example 4-1. However, in example 4-2, the height W1 of eachhoneycomb filter F100 was set at 50 mm, the width W2 was set at 50 mm,and the length L was set at 150 mm. Accordingly, the filtercross-sectional area S was 2,500 mm², and the L/S value was (150/2,500=)0.06 mm/mm².

[0191] In example 4-3, the height W1 was set at 20 mm, the width W2 wasset at 20 mm, and the length L was set at 300 mm. Accordingly, thefilter cross-sectional area S was 4,000 mm², and the L/S value was(300/400=) 0.75 mm/mm².

[0192] In example 4-4, the height W1 was set at 33 mm, the width W2 wasset at 33 mm, and the length L was set at 230 mm. Accordingly, thefilter cross-sectional area S was 1,089 mm², and the L/S value was(230/1089) 0.21 mm/mm².

[0193] In example 4-5 the height W1 was set at 25 m, the width W2 wasset at 25 m, and the length L was set at 300 mm. Accordingly, the filtercross-sectional area S was 625 mm², and the L/S value was (30.0/625=)0.48 mm/mm².

[0194] In example 4-6 the height W1 was set at 22 m, the width W2 wasset at 22 m, and the length L was set at 300 mm. Accordingly, the filtercross-sectional area S was 484 mm², and the L/S value was (300/484=)0.62 mm/mm².

[0195] An experiment was conducted on the five types of assemblies 59 inthe same manner as in example 4-1. As a result, the maximum temperaturedifference ΔT(° C.) was about 0° C. to 10° C., the values of which areextremely small. Further, no cracks were confirmed in any of thehoneycomb filters F100.

COMPARATIVE EXAMPLE 1

[0196] In comparative example 1, the assembly 49 was manufactured in thesame manner as in example 4-1. However, the height W1 of each honeycombfilter F100 was set at 20 mm, the width W2 was set at 20 mm, and thelength L was set at 400 mm. Accordingly, the filter cross-sectional areaS was 1,000 mm², and the L/S value was (400/400=) 1.00 mm/mm².

[0197] An experiment was conducted on the assembly 49 in the same manneras in example 4-1. As a result, the maximum temperature difference ΔT(°C.) was about 30° C. and greater than each embodiment. The length L incomparative example 1 was especially long. Thus, there was a tendency ina temperature difference being produced in the longitudinal direction ofthe filter.

[0198] Further, cracks were confirmed in some of the honeycomb filtersF100, and the honeycomb filters F100 were damaged.

COMPARATIVE EXAMPLE 2

[0199] In comparative example 2, the assembly 49 was manufactured in thesame manner as in example 4-1. However, the height W1 was set at 70 mm,the width W2 was set at 70 mm, and the length L was set at 167 mm.Accordingly, the filter cross-sectional area S was 4,900 mm², and theL/S value was (167/4,900=) 0.03 mm/mm².

[0200] An experiment was conducted on the assembly 49 in the same manneras in example 4-1. As a result, the maximum temperature difference ΔT(°C.) was about 20° C. and greater than each embodiment. The filtercross-sectional area S in comparative example 2 was especially large.Thus, there was a tendency in a temperature difference being produced ina direction perpendicular to the longitudinal direction of the filter.Further, cracks were confirmed in some of the honeycomb filters F100,and the honeycomb filters F100 were damaged.

[0201] The advantages of the ceramic filter assembly 49 of the fourthembodiment will be discussed below

[0202] (1) By setting the ratio L/S between the filter length L and thefilter cross-sectional area in the preferable range, the production of alarge thermal stress is prevented without producing a large temperaturedifference in each of the honeycomb filters F100. This prevents cracksfrom being produced in the honeycomb filters F100 and the honeycombfilters F100 resist being damaged. Due to the increase in the strengthof each honeycomb filter F100, the ceramic filter assembly 49 ismanufactured with superior strength. Further, the employment of theassembly 49 increases the strength of the exhaust gas purificationapparatus 1 and enables usage over a long period.

[0203] The fourth embodiment may be modified as described below.

[0204] (a) As long as the condition of the L/S value being in the rangeof 0.06 mm/mm² to 0.75 mm/mm² is satisfied, the form of the honeycombfilter F100 may be changed to a cylindrical pole-like shape, atriangular pole-like shape, or a hexagonal pole-like shape.

[0205] (b) In addition to using the honeycomb filters F100 as a memberforming the ceramic filter 49, the honeycomb filter F100 may be used asan independent filter.

[0206]FIG. 19 is a perspective view showing a honeycomb filter 59 havinga honeycomb structure according to a fifth embodiment of the presentinvention. FIG. 20 is a cross-sectional view taken along line 20-20 ofthe filter 59 of FIG. 19. FIG. 21 is a cross-sectional view showing amain portion of an exhaust gas purification apparatus.

[0207] It is preferred that the cell density of the honeycomb filter 59be 120/inch² (18/cm²) or greater, and more specifically, be in the rangeof 120 to 180/inch². When the cell density is less than 120, the area ofcontact with the exhaust gas decreases. This lowers the purificationcapability of the honeycomb filter 9.

[0208] It is preferred that the thickness of the cell wall 13 be 0.46 mmor less, and more specifically be in the range of 0.20 to 0.46 mm. Whenthe thickness of the cell wall 13 exceeds 0.46 mm, the opening area ofthe cell decreases and the area of contact with the exhaust gasdecreases. This lowers the purification capability of the honeycombfilter 9. Further, if the cell wall 13 is made thicker than 0.46 mmwhile maintaining the cell opening area, the entire honeycomb filter 9is enlarged.

[0209] It is preferred that the average pore diameter of the honeycombfilter 9 be 5 μm to 15 μm, and further preferred that the average porediameter be 8 μm to 12 μm. If the average pore diameter is less than 5μm, the deposit of particulates clogs the honeycomb filter 9. Thisincreases pressure loss. Thus, the driving performance of the vehiclefalls, fuel efficiency decreases, and the driving feel becomesunsatisfactory. On the other hand, if the average pore diameter exceeds50 μm, fine particles cannot be trapped. This decreases the trappingefficiency and deteriorates the particulate filtering function.

[0210] It is preferred that the porosity of the honeycomb filter 9 be30% to 50%, and further preferred that the porosity be 35% to 49%. Ifthe porosity is less than 30%, the honeycomb filter 9 becomes too dense.This hinders the interior flow of exhaust gas. If the porosity exceeds50%, the number of pores in the honeycomb filter 9 becomes excessive.This may decrease the strength and lower the trapping efficiency of fineparticles.

[0211] Among the pores of the honeycomb filter 9, it is preferred that20% or more be through pores. More specifically, it is preferred that20% to 80% be through pores, and especially preferred that 20% to 50% bethrough bores. A through bore refers to a gap that extends through acell wall 13 and connects adjacent holes 12. If the through pores areless than 20% of the pores, the pressure loss becomes large. Thus, thedriving performance of the vehicle falls, fuel efficiency decreases, andthe driving feel becomes unsatisfactory. On the other hand, if thethrough pores exceed 80% of the pores, manufacture may become difficultand cause stable material supply to be difficult.

[0212] It is preferred that total volume of the honeycomb filter 9 be ¼to 2 times the total displacement of the engine. It is further preferredthat the total volume be ½ to 1.5 times the total displacement. If thevalue is less than ¼, the deposit of particulates clogs the honeycombfilter 9. If the value exceeds 2 times, the honeycomb filter 9 isenlarged. When the honeycomb filter 9 is enlarged, there is a tendencyof the temperature differing between portions of the filter 9 duringcombustion. This increases the thermal stress applied to the honeycombfilter 9 and increases the possibility of the formation of cracks.

[0213] The honeycomb filter 9 is made of sintered porous siliconcarbide, which is a type of sintered carbide. The impurities included inthe sintered porous silicon carbide is 5 wt % or less. It is preferredthat the amount of impurities be 1 wt % or less and it is especiallypreferred that the amount of impurities be 0.1 wt % or less. If theimpurities exceed 5 wt %, impurities concentrate at the grain boundaryof the silicon carbide crystal grains and significantly decreases thestrength at the grain boundary (strength bonding crystal grains). Thismakes the grain boundary vulnerable to breakage. Impurities include Al,Fe, O and free C. Like the honeycomb filter 9, the honeycomb filter 9 ismade of sintered porous silicon carbide.

EXAMPLE 5-1

[0214] Basically, in the same manner as the example 4-1, the throughholes 12 of the molded product were dried with a microwave dryer andthen sealed with a sealing paste made of sintered porous siliconcarbide. Afterward, the drier was used again to dry the sealing paste.Subsequent to the end sealing process, the dried product was degreasedat 400° C. and then sintered for about three hours at 2,250° C. in anargon atmosphere under normal pressure.

[0215] As a result, the produced sintered porous silicon carbidehoneycomb filter 59 had a pore diameter of 10 μm, a porosity of 42%, athrough pore existence rate of 25% relative to the pores, a cell densityof 150/inch², and a cell wall 13 thickness of 0.4 mm. The honeycombfilter 59 had a diameter of 100 mm, a length of 200 mm, and a totalvolume of 2,300 cm³. The total volume refers to the volume obtained bysubtracting the volume of the through holes 12 from the volume of theentire honeycomb filter 59. It is preferred that the thickness of thecell wall 13 be 0.46 mm or less, and more specifically, in the range of0.20 to 0.46 mm.

[0216] Then, the honeycomb filter 59 was wrapped around the honeycombfilter 59. In this state, the honeycomb filter 59 was retained in thecasing. An engine having a displacement of about 3,000 cc was then usedto supply the exhaust gas purification apparatus 1 with exhaust gas at aflow rate of 7 m/sec. In this state, the pressure value of the exhaustgas at the upstream side of the honeycomb filter 59 and the pressurevalue of the exhaust gas at the downstream side were measured. Apressure loss ΔP (mmAq), which is the difference between the values, wasobtained. Further, the amount of soot at the rear side of the honeycombfilter 59 was measured to confirm the amount of particulates that werenot trapped. Further, a certain time period, the honeycomb filter 59 wastaken out and observed with the naked eye to confirm cracks. The resultsare shown in table 1. TABLE 1 Average Existence Soot Amount Total PoreAverage Rate of Pressure Behind Flexural Filter Type of DiameterPorosity Through Loss ΔP Filter Strength Volume Ceramic (μm) (%) Pores(%) (mmAq) (g/km) (Mpa) (cm³) Cracks Example 1 Silicon 10 42 25 80 0.016.5 2300 None Carbide Example 2 Silicon 6 38 30 100 0.01 6.2 2300 NoneCarbide Example 3 Silicon 14 48 45 60 0.015 6.0 2300 None CarbideComparative Silicon 3 10 10 300 0.005 7.2 700 Nohe Example 1 CarbideComparative Silicon 20 70 15 40 0.04 2.5 7000 Confirmed Example 2Carbide Comparative Cordierite 30 20 15 120 0.015 3.1 700 ConfirmedExample 3

[0217] As shown in table 1, the pressure loss ΔP in example 5-1 wasabout 80 mmAq, the value of which is extremely small. The particulateleakage amount was about 0.01 g/km, the value of which is extremelysmall. The honeycomb filter 9 had a flexural strength of 6.5 Mpa and hadan extremely high level of mechanical strength. There were no cracks inthe honeycomb filter 9.

EXAMPLE 5-2, 5-3

[0218] In examples 5-2 and 5-3, the honeycomb filter 59 was manufacturedbasically in the same manner as in example 5-1. However, in examples 5-2and 5-3, only the total volume of the honeycomb filter 59 was the sameas that of example 5-1. The mixture ratio, sintering temperature,sintering time, etc. were changed as described below to adjust the porediameter, porosity, and the through pore existence rate relative to thepores.

[0219] In the produced sintered porous silicon carbide honeycomb filter59 of example 5-2, the pore diameter was 6 μm, the porosity was 32%, andthe through pore existence rate was 30%. The same experiment as that ofexample 5-1 was conducted. The pressure loss ΔP was about 100 mmAq, thevalue of which is extremely small. The particulate leakage amount wasabout 0.01 g/km, the value of which is extremely small. The honeycombfilter 59 had a flexural strength of 6.2 Mpa and had an extremely highlevel of mechanical strength. Further, there were no cracks in thehoneycomb filter 59.

[0220] In the produced sintered porous silicon carbide honeycomb filter59 of example 5-3, the pore diameter was 14 μm, the porosity was 48%,and the through pore existence rate was 45%. In the experiment result ofthis example, the pressure loss ΔP was about 60 mmAq, the value of whichis extremely small. The particulate leakage amount was about 0.015 g/km,the value of which is extremely small. The honeycomb filter 59 had aflexural strength of 6.0 Mpa and had an extremely high level ofmechanical strength. There were no cracks in the honeycomb filter 59.

COMPARATIVE EXAMPLES 1 to 3

[0221] In comparative examples 1 to 3, honeycomb filters weremanufactured basically in the same manner as in example 5-1. However, incomparative example 1, the total volume of the honeycomb filter was 700cm³, which is less than ¼ the displacement (3,000 cc). Further, the porediameter, porosity, and the through pore existence rate relative to thepores was as described below.

[0222] In the produced sintered porous silicon carbide honeycomb filterof comparative example 1, the pore diameter was 3 μm, the porosity was10%, and the through pore existence rate was 10%. In the experimentresult of comparative example 1, the pressure loss ΔP was about 300mmAq, the value of which is extremely large. The particulate leakageamount was about 0.005 g/km, the value of which is extremely small. Thehoneycomb filter had a flexural strength of 7.2 Mpa and had an extremelyhigh level of mechanical strength. There were no cracks in the honeycombfilter.

[0223] In comparative example 2, the total volume of the honeycombfilter was greater than that of examples 1-3 and was 7,000 cm³, which istwo times or greater than the displacement (3,000 cc). Further, in theproduced sintered porous silicon carbide honeycomb filter, the porediameter was 20 μm, the porosity was 70%, and the through pore existencerate was 15%. In the experiment result of comparative example 2, thepressure loss ΔP was about 40 mAq, the value of which is extremelysmall. The particulate leakage amount was about 0.04 g/km, the value ofwhich is extremely small. The honeycomb filter had a flexural strengthof 2.5 Mpa and satisfactory mechanical strength was not obtained. Therewere no cracks in the honeycomb filter.

[0224] In comparative example 3, a cordierite honeycomb filter wasproduced through a known manufacturing method that differs from themanufacturing method of comparative examples 1 and 2. The total volumeof the honeycomb filter was 700 cm³. In the honeycomb filter, the porediameter was 30 μm, the porosity was 20%, and the through pore existencerate was 15%. In the experiment result of comparative example 3, thepressure loss ΔP was about 120 mmAq, the value of which is large. Theparticulate leakage amount was about 0.015 g/km, the value of which islarge. The honeycomb filter had a flexural strength of 3.1 Mpa andsatisfactory mechanical strength was not obtained. There were no cracksin the honeycomb filter.

[0225] Table 1 shows the comparison result of examples 5-1 to 5-3 andcomparative examples 1 to 3, as described above.

[0226] (Experiment Result)

[0227] As apparent from table 1, it was confirmed that exhaust gaspassed smoothly through all of the honeycomb filters 59 in examples 5-1to 5-3. Further, the particulate leakage amount was substantially null,and the required mechanical strength of the honeycomb filter wasobtained. In comparison, the required mechanical strength of thehoneycomb filter was obtained in comparative example 1. However, exhaustgas did not pass smoothly through the honeycomb filter. Further, incomparison example 2, exhaust gas passed smoothly through the honeycombfilter. However, the required mechanical strength was not obtained. Inexample 3, exhaust gas did not pass smoothly through the honeycombfilter, and the required mechanical strength was not obtained.

[0228] The advantages of the honeycomb filter 59 of the fifth embodimentwill now be discussed.

[0229] (1) The sintered porous silicon carbide honeycomb filter 59 isarranged in the casing 8. The honeycomb filter 9 is formed so that theaverage pore diameter is 5 to 15 μm, the average porosity is 30 to 40%,and the through pore existence rate relative to the pores is 20% orgreater. Since the honeycomb filter 9 is not excessively dense, exhaustgas passes smoothly through the interior, and pressure loss isdecreased. This improves fuel efficiency and prevents deterioration ofthe driving feel. Further, since the amount of gaps in the honeycombfilter 9 is not excessive, fine particulates are trapped and thetrapping efficiency is improved. Additionally, even if the honeycombfilter 9 is porous, satisfactory mechanical strength is obtained. Thus,the produced honeycomb filter 9 resists breakage caused by vibrationsand thermal impact.

[0230] (2) The honeycomb filter 9 is formed so that the average porediameter is 8 to 12 μm, the average porosity is 35 to 49%, and thethrough pore existence rate relative to the pores is 20 to 50% orgreater. Thus, the pressure loss is further decreased, and the strengthis increased.

[0231] (3) The end surfaces of the honeycomb filter 9 so that thesealing bodies 14 seal the cells alternately. The number of cells persquare inch is 120 or more, and the thickness of the cell wall 13 is0.46 mm or less. This increases the area of contact with the exhaust gasand increases the purification capability of the honeycomb filter 9.

[0232] (4) The total volume of the honeycomb filter 9 is ¼ to 2 timesthe total displacement of the diesel engine 2. Since the deposit amountof the particulates does not become excessive, clogging of the honeycombfilter 9 is prevented. Further, the honeycomb filter 9 is not enlarged.This prevents the occurrence of temperature differences betweendifferent locations of the honeycomb filter 9 during combustion.Accordingly, the thermal stress applied to the honeycomb filter 9 isdecreased and cracks are not produced.

[0233] The fifth embodiment may be modified as described below.

[0234] (a) The form of the honeycomb filter 9 is not limited to acylindrical pole-like shape and may be changed to a cylindricalpole-like shape, a triangular pole-like shape, or a hexagonal pole-likeshape.

[0235] (b) As shown in FIG. 22, a plurality (16) of honeycomb filters523 may be integrated to manufacture a ceramic filter assembly 521. Ineach polygonal honeycomb filter 523, the average pore diameter is 8 to12 μm, the average porosity is 35 to 49%, and 20 to 50% of the pores arethrough pores. The outer surfaces of the honeycomb filters 523 areconnected to one another by a ceramic seal layer 522.

[0236] In a sixth embodiment, a specific surface area of the particlesforming the cell wall 13 of the honeycomb filter 59 is 0.1 m²/g or more,and more specifically, 0.1 to 1 m²/g. If the specific surface area ofthe cell walls 13 is 0.1 m²/g or less, the deposit of the particulatesclogs the honeycomb filter 59. This increases pressure loss and thusdecreases the fuel efficiency of the vehicle and degrades the feelingdrive. If the specific surface area exceeds 1.0 m²/g, fine particulatescannot be trapped. This decreases the trapping efficiency and causes thefiltering function of the honeycomb filter 59 to become unsatisfactory.

EXAMPLE 6-1

[0237] A honeycomb filter 59 was produced basically in the same manneras that of example 5-1 and the specific surface area of the particlesforming the cell wall 13 was 0.3 m²/g. In example 6-2 and thecomparative example, honeycomb filters 59 were produced basically in thesame manner as example 5-1. The specific surface area of the honeycombfilter 59 of example 6-2 was 0.8 m²/g, and the specific surface area ofthe honeycomb filter 59 of the comparative example was 0.05 m²/g. Ineach of the honeycomb filters 50 of examples 6-1, 6-2 and thecomparative example, the cell density was 150/inch² and the thickness ofthe cell wall 13 was 0.4 mm.

[0238] The honeycomb filter 59 was wrapped by the thermal insulativematerial 10. In this state, the honeycomb filter 59 was retained in thecasing 8. A diesel engine 2 having a displacement of about 3,000 cc wasthen used to supply the exhaust gas purification apparatus 1 withexhaust gas at a flow rate of 9 m/sec. In this state, the pressure valueof the exhaust gas at the upstream side of the honeycomb filter 59 andthe pressure value of the exhaust gas at the downstream side weremeasured. A pressure loss ΔP (mmAq), which is the difference between thevalues, was obtained. The results are shown in table 2. TABLE 2Comparative Example 1 Example 2 Example Specific Surface 0.3 0.8 0.05Area (cm²/g) Particulate 180 120 250 Pressure Loss (mmAq)

[0239] As apparent from table 2, the pressure loss ΔP of the honeycombfilters 59 in example 6-1, example 6-2, and the comparative example was180 mmAq, 120 mmAq, and 250 mmAq, respectively. Accordingly, in examples6-1 and 6-2, a large pressure loss such as that of the comparativeexample was not confirmed.

[0240] The honeycomb filter 59 of the sixth embodiment has theadvantages described below.

[0241] (1) In the honeycomb filter 9, the specific surface area of theparticles forming the cells wall 13 is 0.1 m²/g or greater. Since thehoneycomb filter 9 does not become excessively dense, exhaust gas passessmoothly though the interior, and the pressure loss is decreased.Accordingly, fuel efficiency is improved and degradation of the drivingfeel is prevented. In addition, the upper limit of the specific surfacearea of the particles is 1.0 m²/g. Thus, the gap amount of the honeycombfilter 9 is not excessive and the trapping of fine particles is ensured.This improves the trapping efficiency.

[0242] (2) The sintered silicon carbide cell wall 13 has superior heatresistance. This prevents the cell wall 13 from being deformed or burnedaway. Accordingly, fluid is efficiently purified over a long timeperiod.

[0243] (3) The porous cell wall 13 enables smooth passage of the exhaustgas and further decreases power loss. In addition, the trappingefficiency of particulates is further increased.

[0244] The sixth embodiment may be modified as described below.

[0245] A plurality (16) of honeycomb filters may be integrated tomanufacture a ceramic filter assembly. The specific surface area of thecell wall of each honeycomb filter is 0.1 to 1 m²/g.

INDUSTRIAL APPLICABILITY

[0246] The ceramic filter assembly of the present invention may beapplied to an exhaust gas purification filter of a diesel engine 2, aheat exchange device member, a filter for high temperature fluid or hightemperature vapor, etc.

1. An integral ceramic filter assembly (9) produced by adhering with aceramic seal layer (15) outer surfaces of a plurality of filters (F1),each of which is formed from a sintered porous ceramic body, the ceramicfilter assembly being characterized in that: the seal layer (15) has athickness (t1) of 0.3 mm to 3 mm and a thermal conductance of 0.1 W/mKto 10 W/mk.
 2. The ceramic filter assembly according to claim 1, whereinthe seal layer includes 70 wt % or less of ceramic fiber as a solid. 3.The ceramic filter assembly according to claim 1 or 2, wherein the seallayer includes ceramic fibers having fiber lengths of 100 mm or less. 4.The ceramic filter assembly according to any one of claims 1 to 3,wherein the seal layer includes as a solid 3 wt % to 80 wt % of aninorganic grain.
 5. An integral ceramic filter assembly (29) produced byadhering with a ceramic seal layer (15) outer surfaces of a plurality ofelongated polygonal honeycomb filters (F1), each of which is formed froma sintered porous ceramic body, the ceramic filter assembly beingcharacterized by: round surfaces (18) defined on chamfered corners ofthe outer surface of each honeycomb filter, wherein the round surfaceshave a curvature R of 0.3 to 2.5.
 6. An integral ceramic filter assembly(39) produced by adhering with a ceramic seal layer (15) outer surfacesof a plurality of filters (F1), each of which is formed from a sinteredporous ceramic body, the ceramic filter assembly being characterized by:a ceramic smoothing layer (16) applied to the outer surface of theassembly, which as a whole has a generally circular cross-section orgenerally oval cross-section.
 7. The ceramic filter assembly accordingto claim 6, wherein the smoothing layer has a thickness of 0.1 mm to 10mm.
 8. The ceramic filter assembly according to claim 6 or 8, whereinthe seal layer is thinner than the smoothing layer.
 9. The ceramicfilter assembly according to any one of claims 6 to 8, wherein thesmoothing layer is made from the same material as the seal layer.
 10. Anintegral ceramic filter assembly (49) produced by adhering with aceramic seal layer (15) outer surfaces of a plurality of elongatedhoneycomb filters (F100), each of which is formed from a sintered porousceramic body, the ceramic filter assembly being characterized in that: aratio L/S between a filter length L in a flow direction of a processedfluid and a filter cross-section S in a direction perpendicular to theflow direction is 0.06 mm/mm² to 0.75 mm/mm².
 11. The ceramic filterassembly according to any one of claims 1 to 10, wherein the assembly isa diesel particulate filter.
 12. The ceramic filter assembly accordingto any one of claims 1 to 11, wherein the filter is formed from asintered porous silicon carbide body.
 13. The ceramic filter assemblyaccording to any one of claims 1 to 12, wherein the seal layer includesat least an inorganic fiber, an inorganic binder, an organic binder, andan inorganic grain and is formed from an elastic material obtained bybonding the inorganic fiber, which are three-dimensionally interlinked,and the inorganic grain with the inorganic binder and the organicbinder.
 14. The ceramic filter assembly according to any one of claims 1to 13, wherein the seal layer is formed from 10 wt % to 70 wt % ofsilica-alumina ceramic fiber as a solid, 1 wt % to 30 wt % of silicasol, 0.1 wt % to 5.0 wt % of carboxymethyl cellulose, and 3 wt % to 80wt % of silicon carbide powder.
 15. The ceramic filter assemblyaccording to any one of claims 1 to 14, wherein the filters are arrangedin a state offset from one another in a filter axial direction.
 16. Anintegral honeycomb filter assembly (521) produced by adhering with aceramic seal layer (522) outer surfaces of a plurality of honeycombfilters (523), each of which has a plurality of cells defined by a cellwall (13) and which purifies fluid including particulates with the cellwall, the honeycomb filter assembly being characterized in that: aspecific surface area of grains forming the cell wall is 0.1 m²/g ormore.
 17. An elongated honeycomb filter (F100) formed from a sinteredporous ceramic body, the honeycomb filter being characterized in that: aratio L/S between a filter length L in a flow direction of a processedfluid and a filter cross-section S in a direction perpendicular to theflow direction is 0.06 mm/mm² to 0.75 mm/mm².
 18. A honeycomb filter(F100) formed from a sintered porous ceramic body, the honeycomb filterbeing characterized in that: an average pore diameter of the honeycombfilter is 5 to 15 μm, an average porosity is 30 to 50%, and thehoneycomb filter has 20% or more of through pores.
 19. The honeycombfilter according to claim 18, wherein the average pore diameter is 8 to12 μm, the average porosity is 35 to 49%, and the ratio of through poresis 20 to 50%.
 20. The honeycomb filter according to claim 18 or 19comprising a plurality of cells including a first cell having a firstend surface sealed by a sealing body (14) and a second cell adjacent tothe first cell by way of a cell wall and having a second end surfaceopposite to the first end surface sealed by a sealing body, wherein thecell number per square inch is 120 or more, and the thickness of thecell wall defining the cells is 0.46 mm or less.
 21. A honeycomb filter(59) having a plurality of cells defined by a cell wall (13) andpurifying fluid including particulates with the cell wall, the honeycombfilter being characterized in that: a specific surface area of grainsforming the cell wall is 0.1 m²/g or more.
 22. The honeycomb filteraccording to claim 21, wherein the cell wall is formed from a sinteredsilicon carbide body.
 23. The honeycomb filter according to claim 21 or22, wherein the cell wall is formed from a porous body.
 24. An exhaustgas purification apparatus including a honeycomb filter (59) formed froma sintered porous ceramic body and arranged in a casing (8) that islocated in an exhaust gas passage of an internal combustion engine (2)to eliminate particulates included in exhaust gas, the exhaust gaspurification apparatus being characterized in that: an average porediameter of the honeycomb filter is 5 to 15 μm, an average porosity is30 to 40%, and the honeycomb filter has 20% or more of through pores.25. The exhaust gas purification apparatus according to claim 24,wherein the average pore diameter of the honeycomb filter is 8 to 12 μm,the average porosity is 35 to 49%, and the honeycomb filter has 20 to50% or more of through pores.
 26. The exhaust gas purification apparatusaccording to claim 24 or 25 comprising a plurality of cells including afirst cell having a first end surface sealed by a sealing body (14) anda second cell adjacent to the first cell by way of a cell wall andhaving a second end surface opposite to the first end surface sealed bya sealing body, wherein the cell number per square inch is 120 or more,and the thickness of the cell wall defining the cells is 0.46 mm orless.
 27. The exhaust gas purification apparatus according to any one ofclaims 24 to 26, wherein the total volume of the honeycomb filter is ¼to 2 times the total displacement of the internal combustion engine.